Quasi-resonant Flyback (QR Flyback) is widely used in low power application. It can achieve zero-voltage-switch (ZVS) of primary switch at low line input condition, but for high line input condition, the turning on loss of primary switch is still considerable despite its valley-switching operation. To achieve higher power density of the switching mode power supply, higher switching frequency is one of the development trend. As frequency rises, switching loss of the primary switch of the Flyback converter rise in direct proportion, which leads to serious efficiency drop, especially at high line input condition. To solve this problem, ZVS Flyback under the entire input voltage range was proposed such as active clamp Flyback.
Although the active clamp Flyback can achieve ZVS of the primary switch, an additional switch is needed, thereby increase cost.
Another more economical solution is also proposed. Please refer to FIGS. 1 and 2. FIG. 1 is a circuit diagram of a QR Flyback in the prior art; and FIG. 2 is a voltage timing diagram of FIG. 1. As shown in FIGS. 1 and 2, a reverse current is generated in a secondary coil through the extended conducting time of a secondary rectifier, and, after turning off the rectifier, the current participates in the resonance of magnetizing inductor Lm and the parasitic capacitor CEQ of the primary switch to achieve ZVS of the primary switch.
To apply the above method, there is a limiting condition: the circuit must operate in a BCM (boundary conduction mode) within the entire input voltage range and the entire load range; if it operates in a DCM (discontinuous conduction mode), that method cannot be adopted. This has a serious impact on the light load efficiency at high line input condition. The reason is as follows: according to the working principle of the BCM of a QR Flyback, in the case of the same load, the higher the input voltage leads to the higher the operating frequency; in the case of the same input voltage, the lighter the load leads to the higher the operating frequency. Therefore, under the condition of high-voltage input and light load, the operating frequency will become very high, and the switching loss will seriously affect the efficiency. In practical applications, in order to enhance efficiency, it is necessary to set the operating frequency of a power supply within a reasonable range. In the case of light load, the operating mode of the circuit will always be switched from the BCM to the DCM; during high-voltage input, even in the case of full load, it may also choose to work in the DCM to enhance efficiency. In this operating mode, the existing method of achieving ZVS of a primary switch by means of the extended conducting time of a secondary rectifier does not apply. Therefore, an optimizing control strategy for the existing topology of QR Flyback is proposed in the present invention.